1. Field of the Invention
This invention relates to a method for the fabrication of electrooptically active fiber segments that can be readily integrated into optical fiber lines, with applications in high-speed modulators, electric field sensors, or optical mixers.
2. Description of the Related Art
This invention is related to U.S. Pat. No. 5,239,407, Method and Apparatus for Creating Large Second-Order Nonlinearities in Fused Silica and U.S. Pat. No. 5,247,601, Arrangement for Producing Large Second-Order Optical Nonlinearities in a Waveguide Structure including Amorphous SiO.sub.2. These patents are hereby incorporated by reference.
Packaging costs associated with opto-mechanical coupling of discrete optical components are a major part of the cost for advanced optoelectronic systems. For example, for many high-speed fiber communications systems, the output of a diode laser must be coupled into a single-mode optical fiber, the fiber must then be coupled to a LiNbO.sub.3 waveguide modulator whose output is again coupled into a fiber. Discrete optical components, e.g., graded-index lenses or micro-lenses are required at each coupling node to adapt the very different mode profiles and spatial extents of the diode laser and modulator waveguide modes to the fiber mode. Tolerances are fractions of a micrometer to ensure minimal coupling losses and extensive active alignment (optimizing coupling with the laser on) is typically required. Throughput and yield are both limited by the requirement of keeping the system stable while the bonding agents cure. If a modulator could be produced that was integrated into the fiber, the manufacturing and packaging costs associated with these high-speed fiber communications systems would be substantially reduced.
Electric field sensors are another potentially attractive application of electrooptically active fibers. The electric power industry has a need for remote sensors to monitor high voltage power systems. Integrating electrooptically active fiber sensors with Bragg reflector gratings is a very attractive alternative to currently available sensors that will have a major economic impact.
A third potential area of application of electrooptically active fibers is frequency mixing (i.e., second harmonic and sum frequency generation to reach shorter wavelengths than the starting wavelengths and difference frequency generation to reach longer wavelengths). This would enable the extension of the utility of high power diode lasers which are today confined to the wavelength range from roughly 700 nm to 1 micrometer. Applications include high-density optical recording, displays, and spectroscopic sensors. These nonlinear mixing processes require both a second-order nonlinearity, the same order nonlinearity that gives rise to the electrooptic effect, as well as a phase matching technique to ensure that the nonlinear mixing stays coherent along the active length of the fiber. Previous work (X.-C. Long, R. A. Myers and S. R. J. Brueck, Measurement of the linear electrooptic coefficient in poled amorphous silica, Optics Letters 19, 1820 (1994); X.-C. Long, R. A. Myers and S. R. J. Brueck, Measurement of linear electrooptic effect in temperature/electric-field poled optical fibers, Electronics Letters 30, 2162 (1994)) has shown that the second-order nonlinearity and the electrooptic effect induced in germanosilicate glasses arise from the same electronic processes and are closely related. The required phase matching is most conveniently achieved by quasi-phase matching in which the nonlinearity is alternately turned on and off each coherence length (i.e., the length over which the phases of the fundamental and second harmonic fields are shifted by .pi. because of their different velocities). More complex poling patterns may be desirable to tailor the phase matching bandwidth for specific applications.
There has been extensive work on integral fiber lasers involving doping the fiber with an appropriate chromophore (typically a rare earth element such as Er for the important 1.55-.mu.m telecommunications band) and integrating mirrors onto the fiber either by polishing and coating the fiber ends or by using photogenerated gratings. (See K. O. Hill, B. Malo, F. Bilodeau and D. C. Johnson, Photosensitivity in Optical Fibers, in Annual Review of Materials Science, Vol. 23, pp. 125-157, R. A. Laudise and E. Snitzer, eds. (Annual Reviews, Palo Alto, 1993) for a recent review of photogenerated gratings and their application to fiber lasers.) These fiber lasers cannot be modulated at communications rates (Mb/s-Gb/s) by turning the pumping beam on and off because of the inherent long lifetimes of the dopants (typically.about.ms). Hence, an external modulator is essential for communication applications, a modulator internal to the cavity can be used for Q-switching and mode-locking of the laser output to produce short, high power pulses.
Clearly, integration of the modulator into a fiber that can simply be spliced onto the laser fiber in a simple, inexpensive, rapid process would be the optimal packaging solution. Such a high-speed modulator integrated into a fiber is a very important element that has yet to be realized. The discovery by our group at the University of New Mexico of a stable second-order nonlinearity induced in SiO.sub.2 materials has led to a great deal of work aimed at establishing a practical geometry for both waveguide (U.S. Pat. No. 5,247,601) and fiber :modulators (see X.-C. Long, et al and P. G. Kazansky, P. St. J. Russell, L. Dong and C. N. Pannell, Pockels effect in thermally poled silica optical fibers, Electronics Letters 31, 62 (1995)).
Recently, a group at the Australian Fiber Optic Research Center has demonstrated (T. Fujiwara, et al, Electrooptic modulation in germanosilicate fibre with UV-excited poling, Electronics Letters 31,573,30 Mar. 1995) a significant improvement in the effective electrooptic coefficient of a fiber with two innovations: a) use of an ultraviolet beam along with an applied electric field to produce the poling in contrast to the use of high temperatures (.about.100.degree.-300.degree. C.) under an applied electric field, and b) provision for wire electrodes internal to the fiber to increase resistance to breakdown during the poling and to provide a better overlap between the nonlinearity and the optical mode volume. They were able to achieve an electrooptic (r) coefficient of 6 pm/V. This is significantly larger than previously reported (0.05 pm/V) and is sufficiently large for practical application. Their technique, however, has a number of drawbacks. Specifically, the fiber is drawn from a preform with two holes for electrode wires that are to be inserted following the fiber drawing. This wire insertion is a difficult manufacturing step, comparable to the difficulty and expense of coupling discrete optical components. To avoid breakdown, one wire is inserted from each end of the fiber. This means that the modulation frequency is limited to low values since a high-speed traveling wave geometry is not possible. Furthermore, splicing to either end of the fiber is not possible because of the electrodes, again requiring discrete optical system alignment for coupling into the fibers, negating much of the advantage of an electrooptically active fiber segment--the fabrication of an all-fiber active device.
Clearly, there is a need for an electrooptically active fiber segment that can be simply spliced with other fiber components onto a laser fiber to significantly reduce the manufacturing and packaging costs.